1. Field of the Invention
The present invention relates to an image forming apparatus using an electronic photographic system such as a copier, printer, facsimile machine, or a combination of these, and more specifically relates to an image forming apparatus and process cartridge using a two-component developer comprising toner and carrier.
2. Description of the Related Art
Image forming apparatuses such as color copiers and color printers that conduct a developing process step using a two-component developer comprising toner and carrier (including developers that also comprise other additives, etc.) have been disclosed in the past such as Japanese Unexamined Patent Application Publication No. 2004-212560. Disclosed in this publication is an image forming apparatus that in addition to using a developing system that prints only the DC developing bias using the two-component developer, also used a small diameter carrier as the carrier in the two-component developer to heighten image quality. This technology has the object of reducing the generation of carrier adhesion, and of reducing generation of image blotching and characters with missing periphery, etc. Specifically, using a small particle size carrier with a weight average particle size of 20 to 60 μm optimizes the static resistance and saturation magnetization of the carrier.
Moreover, disclosed in Japanese Unexamined Patent Application Publication No. H7-140723 is a technology that is an image forming apparatus using a two-component developer having the object of preventing carrier adhesion and stabilizing the amount of static electricity, and comprising carbon black as the resistance adjuster in the carrier.
Nonetheless, the conventional technologies described above have had difficulties both with the operating life of the two-component developer and with preventing toner scattering. That is, from the perspective of conserving raw materials and reducing the running costs, it is important to extend the operating life of the two-component developer (the operating live of the carrier) used in the developing process. Generally, after initially rising and reaching a peak, the charge capacity of the carrier gradually decreases corresponding to passage of agitation time (or number of printed images). Then, when the charge capacity of the carrier falls below a specified amount, the end of the operating life is reached. The amount of charge of the toner (toner charge) also initially increases and gradually decreases over time corresponding to the changes over time of this kind of carrier charge capacity.
Meanwhile, the size of the initial charge capacity of the carrier can be freely set depending on the adjustment of components. Consequently, it is possible to anticipate the changes of carrier charge capacity over time as described above by setting a large initial carrier charge capacity. However, in this case, the toner concentration (the percentage of toner in the developer) is increased by increasing the amount of initial toner charge, and toner scattering is prone to occur. When toner scattering occurs, the interior of the image forming apparatus may become stained, or staining on the output image may also occur.
The technology disclosed in the previously described Japanese Unexamined Patent Application Publication No. 2004-212560 sets small diameter carrier conditions (static resistance, saturation magnetization, etc.) for reducing generation of image blotching and characters with missing periphery, as well as carrier adhesion. However, there is the problem that extended developer operating life has not been fully achieved.
Moreover, the technology disclosed in the previously described Japanese Unexamined Patent Application Publication No. H7-140723 can expect the effect of stabilizing the amount of charge over time because carbon black is comprised in the carrier as a resistance adjuster. However, if about 300,000 pages of printed images are assumed as the operating life, a problem is posed by the concern that the carbon black comprised in the carrier can migrate to the toner over time and promote deterioration of the carrier.